In a conventional boost converter, an input voltage is coupled via an inductor to a switch, typically a MOSFET, and a diode and a capacitor in series are coupled in parallel with the switch, an output voltage of the converter being derived from the capacitor. In the absence of a transformer, the output voltage is greater than the input voltage. The switch is alternately opened and closed, typically at a high frequency and with a controlled duty cycle.
An increasingly important application of boost converters is for power factor correction (PFC) in so-called offline power supply arrangements for consumer electronics equipment. In such arrangements typically a rectified AC power supply is converted by a boost converter to a high output voltage to provide a near-unity power factor; the output voltage can be used directly or converted by one or more other power converters to one or more AC and/or DC voltages for use.
Operation of a boost converter in discontinuous current mode (DCM), in which the converter switch is turned on when the inductor current is zero, has the results that the peak current is twice the average current and the inductor current has large swings, requiring a relatively large core involving increased losses. With increasing converter power levels, for example for power levels greater than about 200 or 300 W as may be required for a boost converter for PFC, it is preferable to operate the boost converter in continuous conduction mode (CCM), in which the converter switch is turned on before the inductor current has fallen to zero. A boost converter operated in CCM has relatively smaller inductor current swings and peak current.
In consequence, the diode of the boost converter, referred to as the boost diode, is required to have a very fast reverse recovery, especially in view of the typical high output voltage of a boost converter used for PFC. For example, such a boost converter may typically be desired to operate with a peak input voltage up to about 360V, and the output voltage may conveniently be selected to be about 380 to 400V. During the reverse recovery period, immediately after the converter switch is turned on so that the diode is reverse biased, after having been forward biased and conducting the non-zero inductor current, the diode is still conductive due to carriers in the diode junction region, and very large reverse currents can flow, substantially increasing the stress and power loss in the converter switch.
The diode of a boost converter used for PFC can be based on silicon carbide semiconductor technology, but such diodes may have a cost of the order of ten times that of silicon diodes. Even with a diode that does not exhibit reverse recovery behaviour, the converter switch is turned on and off with the full current of the inductor flowing, resulting in substantial switching losses.
In order to reduce these disadvantages, it is known to provide more complex arrangements of a boost converter incorporating an additional or auxiliary switch. Examples of such converters are described in Bassett et al. U.S. Pat. No. 5,446,366 issued Aug. 29, 1995 and entitled “Boost Converter Power Supply With Reduced Losses, Control Circuit And Method Therefor”; Jovanovic U.S. Pat. No. 5,736,842 issued Apr. 7, 1998 and entitled “Technique For Reducing Rectifier Reverse-Recovery-Related Losses In High-Voltage High Power Converters”, and in Jang et al. U.S. Pat. No. 6,051,961 issued Apr. 18, 2000 and entitled “Soft-Switching Cell For Reducing Switching Losses In Pulse-Width-Modulated Converters”.
The additional complexities and additional switch of such known converters add to their cost, as well as to the complexity and cost of the control circuit which must be provided for controlling the switches of the boost converters.
It is also known from Farrington et al. U.S. Pat. No. 5,550,458 issued Aug. 27, 1996 and entitled “Low-Loss Snubber For A Power Factor Corrected Boost Converter” to provide a boost converter with a snubber to reduce diode reverse recovery and switching losses without providing the converter with an additional switch. In this converter a snubber inductor is connected in series with the boost diode, and a resistor in series with a snubber diode is connected in parallel with the series-connected boost diode and snubber inductor. This arrangement has the disadvantage of requiring a further diode connected to the junction between the boost diode and the snubber inductor to prevent ringing of the voltage across the boost diode when the switch is on, with a resulting current circulating through the snubber inductor, this further diode, and the converter switch. This reference also discloses a similar snubber arrangement applied to a buck converter.
Another boost converter with a snubber circuit, having the disadvantage of further complexity, is known from Kim U.S. Pat. No. 5,633,579 issued May 27, 1997 and entitled “Boost Converter Using An Energy Reproducing Snubber Circuit”.
There remains a need to provide a power converter, such as a boost converter or a buck converter, with reduced switching and/or reverse recovery losses using a relatively simple arrangement without an additional switch.